1. Field of the Invention
The present invention relates to a method for manufacturing a titanium alloy sheet, and particularly, to a method for efficiently manufacturing a titanium alloy sheet excellent in surface conditions and workability.
2. Related Art Statement
A titanium alloy sheet, particularly an .alpha.+.beta. type titanium alloy sheet is conventionally manufactured by a pack-rolling using a plate mill as disclosed in Japanese Patent Provisional Publication No. JP-A-63-76,706 (hereinafter referred to as the "prior art 1").
The pack-rolled titanium alloy sheet is conventionally manufactured by covering at least upper and lower surfaces of a titanium alloy slab with mill scale or a titanium alloy slab subjected to a surface treatment such as descaling with carbon steel plates, and hot-rolling the titanium alloy slab thus covered with the carbon steel plates.
Another conventional pack-rolling comprises the steps, as shown in FIGS. 1 and 2, of covering upper and lower surfaces and peripheral side surfaces of a titanium alloy slab 4 with mill scale or a titanium alloy slab 4 subjected to a surface treatment such as descaling with an envelope comprising carbon steel plates 1 (hereinafter referred to as the "carbon steel envelope") to prepare an assembled slab, providing deaerating holes 5 for discharging air in the interior of the assembled slab during the hot-rolling in the open air, or slits having a function equivalent to the above holes 5, on the carbon steel envelope, and then hot-rolling the titanium alloy slab thus covered with the carbon steel envelope, i.e., the assembled slab. In order to prevent bonding between the carbon steel envelope and the titanium alloy slab housed therein, a release agent is disposed therebetween when preparing the foregoing assembled slab. The above-mentioned assembled slab is prepared by welding together the carbon steel plates 1 on the upper surface, the lower surface and the peripheral side surfaces in the open air along welding grooves 6 provided between the upper and the peripheral side carbon steel plates and between the lower and the peripheral side carbon steel plates.
In general, the temperature of a titanium alloy slab remarkably decreases during the hot-rolling according as the thickness thereof decreases, resulting in a lower workability. According to the method of the prior art 1, since the titanium alloy slab is covered with the carbon steel envelope, there is only a slight decrease in the temperature of the titanium alloy slab during the hot-rolling, thus making it possible to roll the titanium alloy slab within a high temperature range. It is consequently possible to manufacture a titanium alloy sheet by the use of an ordinary hot-rolling mill such as a plate mill.
Further, a commercially pure titanium sheet and a titanium alloy sheet have anisotropy in strength. According to the method for manufacturing a titanium alloy sheet of the prior art 1 based on the pack-rolling using a plate mill, a cross-rolling can be applied, thus permitting reduction of anisotropy in strength of the commercially pure titanium sheet and the titanium alloy sheet.
The U.S. Pat. No. 5,121,535 issued on Jun. 16, 1992 (corresponding to Japanese Patent Provisional Publication No. JP-A-2-263, 504) discloses a method for shaping metal sections of reactive metals comprising the steps of (hereinafter referred to as the "prior art 2"):
In the foregoing method of the prior art 2, said encapsulating step comprises the following sub-steps of: (a) preparing a metal frame of said second metal, said metal frame having a window therein, (b) mounting said first metal in said window in said metal frame, (c) interleaving said metal frame and said first metal between two layers of said second metal, thereby forming a laminate metal assembly, and (d) welding said two layers of said second metal to said metal frame, and wherein said two layers of said second metal include surface depressions, and said release agent is disposed in said surface depressions.
In the method of the prior art 2, furthermore, the sub-step of welding the two layers of the second metal to the metal frame comprises an electron beam welding under a vacuum atmosphere.
When applying the method of the prior art 2 to the manufacture of a titanium alloy sheet, the metal assembly under a vacuum atmosphere, which houses the titanium alloy slab therein is hot-rolled. It is therefore possible to restrain the formation of a thick and tight oxide scale on the surface of the titanium alloy slab during the heating and during the hot-rolling of the metal assembly in the open air. It is accordingly possible to omit or simplify an excessive polishing or grinding step by means of a grinder, which serves also for a thickness adjustment, or a shot-blasting step or a pickling step, for removing the thick and tight oxide scale.
Furthermore, according to the method of the prior art 2, which adopts the electron beam welding under a vacuum atmosphere, the interior of the metal assembly tack-welded in the open air can be made a into vacuum atmosphere in a vacuum chamber within a relatively short period of time. More specifically, it is possible to achieve a vacuum atmosphere within a relatively short period of time in the interior of the metal assembly, which interior has a small space because of the titanium alloy slab housed therein, and accordingly has a large deaeration resistance.
The prior arts 1 and 2 have however the following problems. In the prior art 1, in which the hot-rolling is carried out in the open air, an oxide scale and/or a deteriorated layer, in which a large quantity of oxygen is dissolved in the form of solid-solution, are formed during the heating or during the hot-rolling of the assembled slab not only when a slab in the assembled slab is a titanium alloy slab with mill scale, but also even when the slab is a titanium alloy slab subjected to a surface treatment such as descaling. The above-mentioned oxide scale and deteriorated layer cause deterioration of surface conditions of the titanium alloy sheet as a product and a serious decrease in material properties such as bendability. It is therefore necessary to remove these oxide scale and deteriorated layer.
Available methods for removing the oxide scale and the deteriorated layer include a method of polishing and grinding the surface of the titanium alloy sheet by means of a grinder or the like to remove the oxide scale and the deteriorated layer, and a method of using a shot-blasting and a pickling to remove the oxide scale a the deteriorated layer. According to the method of removing the oxide scale and the deteriorated layer by means of a grinder or the like, thickness of the sheet can be simultaneously adjusted. It is therefore possible to manufacture a titanium alloy sheet having a high thickness accuracy and containing only a small strain. A problem is however that, because the titanium alloy sheet having a low machinability and a large area is to be polished or ground, the foregoing descaling step requires a long period of time and the manufacturing cost is higher.
According to the method of removing the oxide scale and the deteriorated layer through the shot-blasting and the pickling, on the other hand, it is possible to complete the descaling in a short period of time. A problem is however that strain occurs by the shot-blasting in the titanium alloy sheet. According to the method of removing the oxide scale and the deteriorated layer only through the pickling, omitting the shot-blasting, on the titanium alloy sheet manufactured by the hot-rolling in the open air, it is impossible to completely remove the thick and tight oxide scale and the deteriorated layer formed during the heating and the hot-rolling of the titanium alloy slab. A problem is therefore that material properties such as bendability of the titanium alloy sheet are seriously decreased.
When subjecting the metal assembly of which the interior is in a vacuum atmosphere to the hot-rolling as in the prior art 2, various problems as in the prior art 1 caused by the thick and tight oxide scale and the deteriorated layer, in which a large quantity of oxygen is dissolved in the form of, can be solved. However, a new surface is produced on the surface of the titanium alloy slab during the above-mentioned hot-rolling under a vacuum atmosphere, and bonding occurs between the non-reactive second metal in the prior art 2 and the reactive first metal in the prior art 2 (i.e., the titanium alloy slab), which compose the metal assembly, or between titanium alloy sheets when two or more piled titanium alloy slabs are encapsulated in the second metal. In order to prevent the foregoing bonding, a release agent is used. However, the release agent comes off during preparing the metal assembly after applying the release agent, and during hot-rolling, thus causing the aforesaid bonding, or the releasing agent coheres, thus causing dents or the like on the surface of the titanium alloy sheet. This results in a problem that the surface conditions of the titanium alloy sheet are deteriorated so seriously that the manufactured titanium alloy sheet cannot be used as a product. According to the method of the prior art 2, furthermore, a special working step is required for providing depressions in the non-reactive second metal. Because the release agent is disposed in the depressions of the second metal, the metal assembly can receive only a sheet of the reactive first metal. This makes it impossible to adopt an efficient method of, for example, forming a plurality of sheets of the reactive first metal by means of a single run of hot-rolling.
Under such circumstances, there is a strong demand for development of a method for efficiently manufacturing a titanium alloy sheet excellent in surface conditions and workability, but such a method has not as yet been proposed.